The present invention generally relates to read-only memories and more particularly to a read-only memory wherein the memory cells thereof incorporate relatively high or relatively low capacitances representing bits of information permanently stored therein.
Read-only memories find considerable application in modern electronic equipment, such as computers, and micro-processors. Read-only memories provide the function of permanently storing information in bits of binary data. The binary data may be used, for example, to define fixed program instructions in a processor or computer of electronic systems. The type of fixed instructions determines the function to be performed by the system. For example, one set of stored instructions may cause a system to act as a controller for manufacturing operations. Another set of instructions may cause the system to act as calculator. Still another set of instructions may cause a system to function as a switchboard in a communications or information network. Obviously, read-only memories take an important role in the modern, electronic technology of today.
Read-only memories are generally implemented in integrated circuit form. One form of read-only memory utilizes an MOS transistor at each memory cell. A completed MOS transistor may be used to represent a stored binary "1" while an incomplete MOS transistor at a memory cell may be used to represent a stored binary "0". Whether such an MOS transistor is completed or not depends on whether an effective gate electrode is provided the transistor during the integrated circuit processing. An effective gate electrode is provided by forming a thin oxide region between the transistor source and drain electrodes and a row address line overlying the transistor source and drain. The oxides are first formed to be relatively thick and a selective etching process is used to reduce the oxide thickness to a thin oxide in selected areas. When the row address lines are formed, they become the gate electrodes for those transistors having thin oxides selectively etched over their gate and source electrodes.
While the foregoing read-only memories have found wide acceptance in the art, there remains room for improvement. For example, a selective-etch process to form the thin oxide regions must be accurately and carefully controlled to achieve the desired oxide thicknesses to assure that only the desired transistors to be completed are, in fact, completed. Also, because source and drain electrodes must be formed, small processing feature sizes must be used to obtain acceptable cell size. Cell size is particularly important because it determines memory packing density. The memory packing density of the foregoing type of read-only memory has been limited by feature size.
The read-only memory of the present invention overcomes the aforementioned deficiencies in the prior art. The read-only memory of the present invention does not require a selective etch to provide selective areas of thin oxides, and does not require the formation of transistors. The read-only memory therefore results in memory cells of smaller size and thus, increased packing density.